This invention relates in general to the manufacture of multilayer capacitors and, more specifically, to improved methods of manufacturing multilayer capacitors with improved interlayer bonding.
Conventional multilayer capacitors generally consist of a number of alternate layers of conductive metal electrodes and dielectric layers, all connected in parallel so as to provide an increase in the electrical capacitance for a given area. This structure is generally referred to as a monolithic construction of electrodes and dielectrics or as a monolithic capacitor. The dielectric may be an insulating synthetic resin, a ceramic material or other insulator. A variety of conducting materials, typically metals, may be used in the electrodes.
At the present time, the electrodes are generally formed on a dielectric substrate through a "silk screen" printing process in which a type of printing ink comprising finely divided precious metal particles (typically having diameters of about 1 micrometer) dispersed in a resinous carrier are forced through a screen stencil onto the substrate. Because of the particulate nature of the conductor and the non-conductive resin matrix, the resulting layer does not have an optimum high conductivity. As the layer is made thinner than about 0.001 conductivity decreases to an undesirable degree becoming unusable below about 0.0006 inch. The decrease in conductivity beyond the linear decrease due to thinner deposits, is basically due to two effects, (1) roughness of the surface of the ceramic substrate absorbs approximately 30% of the metal contained in the 0.0006 inch wet deposition in filling in the roughness before any appreciable conductivity takes place and (2) the conductivity of the final form of a silk screened electrode is effectively the same as that of highly compacted metallic powder which in general exhibits considerably less conductivity than solid metal. These effects severely limit conductivity of thinner layers than 0.0006 inch of metallic particle ink electrodes.
While silk screening remains the customary method of fabricating multilayer capacitors, attempts have been made to use other methods for applying a conductive layer to form the electrodes.
As described by Behn et al. in U.S. Pat. No. 4,376,329, simple capacitors have been made by vapor deposition of a metal such as aluminum onto a substrate, followed by forming a layer of synthetic resin by gas polymerization, then vapor depositing another metal layer. This method is complex and cannot effectively produce multilayer ceramic capacitors comprising alternate layers of metal and ceramic dielectrics at high rates.
Another method of producing laminated capacitors is described by Behn in U.S. Pat. Nos. 4,378,382 and 4,508,049. Here, carriers are located in recesses in a drum, which is rotated to move the carriers alternately through vacuum chambers which deposit a metal such as aluminum, then a synthetic resin dielectric, by vacuum deposition. This is another complex system, requiring complex seals where the drum enters and leaves the vacuum chambers. This method does not seem adaptable to ceramic dielectrics and precious metal electrodes in systems for multilayer ceramic capacitors.
Glow discharge sputtering deposition facilitates a greater degree of conductivity for a given thickness of metal, thus allowing the use of thinner electrodes and thinner dielectric substrates. However, it has been found that sputtering of electrodes directly onto a substrate made up of ceramic particles and a resinous binder must be carried out at a very slow rate to avoid overheating the ceramic substrate. This heating is derived from the plasma which is a function of the sputtering system. Over heating of the ceramic/resin substrate inhibits the laminating process in which many ceramic substrates bearing sputtered electrodes are pressed together to form interlaminated metallic electrode and ceramic substrate sandwiches. Higher sputtering rates do not otherwise damage the substrate, but do prevent effective lamination.
Thus, there is a continuing need for methods of producing high temperature resistant multilayer capacitors using thin ceramic dielectric substrates and thin precious metal electrodes well laminated together to form monolithic capacitors.